The most abundant source of transforming growth factor.beta. (TGF-.beta.) in the body is bone. The process of new bone formation in young adults, which involves the removal of bone by osteoclasts (OC) and replacement of that bone by osteoblasts (OB), appears to be tightly regulated by locally produced factors. TGF-.beta. is a prime candidate for one of these factors as both OB and OC are known to produce TGF-.beta..
The production and activation of TGF-.beta. is regulated by estrogen, parathyroid hormone, glucocorticoids, and other bone regulatory agents including TGF-.beta. itself (Oursler et al, Endocrinology, 129, 3313 (1991); Oursler et al., Endocrinology, 133, 2187 (1993); Subramaniam et al., J. Cell. Biochem., 51, 52 (1995)). Disregulation of TGF-.beta. production and/or activation may be involved in bone diseases (Mundy, In: Bone Remodeling and its Disorders, Dunitz (ed.), U.K., pp. 56-65 (1995); Roberts et al., Growth Factors, 8, 1 (1993)). Thus, TGF-.beta. is thought to play an integral role in human bone cell physiology as a growth and/or differentiation-inducing factor (Mundy, cited supra).
In vitro, TGF-.beta. has been shown to have strong mitogenic activity in osteoblasts (Robey et al., J. Cell Biol., 105, 457 (1987)). Moreover, TGF-.beta. is known to have multiple effects on bone-specific genes. For example, TGF-.beta. has been shown to increase type I collagen, osteopontin, and alkaline phosphatase synthesis, and decrease osteocalcin synthesis (Noda et al., J. Cell. Physiol., 133, 426 (1987); Pfeilschifter et al., Proc. Natl. Acad. Sci USA, 82, 2024 (1987); Wrana et al., J. Cell Biol., 106, 915 (1988); Noda et al., J. Biol. Chem., 263, 13916 (1988)). The regulation of these genes, as well as c-fos and jun-B (Subramaniam et al., cited supra), by TGF-.beta. has been measured after 2-24 hours of TGF-.beta. treatment. However, the early mechanism of action of TGF-.beta. in bone-derived cell cultures has not been characterized.
Thus, there is a need to identify and isolate genes that are expressed immediately or very soon after exposure of bone tissues to growth factor. There is also a need to identify and isolate genes that encode factors which are involved in the regulation of bone growth and differentiation.